Landing gear for semitrailers have heretofore been and will presumably continue to be a high volume product, with estimated sales in the United States normally exceeding 300,000 sets per year and generating income of more than 50 million dollars. The high demand for landing gear is directly related to the use of semi-trailers pulled by truck tractors as a primary means of shipping goods in the United States.
A conventional two-speed landing gear consists of a pair of gear-driven, laterally spaced, telescopic legs which are mounted on and depend from a front end of the semitrailer which engages the truck tractor.
Landing gear, and particularly two-speed landing gear, are used in association with semitrailers in the following manner. A typical scenario begins with the operator of a truck tractor dropping off the semi-trailer at a location such as a dock for loading or unloading of the semitrailer. This semitrailer is retrieved when loading or unloading is completed, often by another tractor. When disconnecting after positioning the semitrailer in the desired location, the truck operator manually turns a crank handle to extend the legs until the legs engage the ground. The operator typically uses the high gear or high speed of the two-speed gear assembly to quickly extend the landing gear legs from the retracted position to the extended position where the legs contact the ground. It should be noted that when the landing gear assembly is engaged in high gear, quick traversal of a vertical distance can be accomplished due to the low ratio of crank handle turns to inches of vertical leg travel distance afforded by the high gear, which ratio usually is from about 2 to about 5 depending on the unit being utilized. However, the tradeoff for such a low ratio achieved in high gear is a lower mechanical advantage than that which can be achieved in a low gear. In practical terms, this means that the truck operator cannot raise or lower a load in high gear. In low gear the ratio of crank handle turns to distance traveled is higher, usually from about 15 to about 50, but a higher mechanical advantage is enjoyed. This enables the truck operator to raise or lower loads in low gear that could not be moved in high gear, albeit at a slower pace than if such loads could be moved in high gear. In order to pull the truck tractor away from the stationary semitrailer, the operator must disengage the depending kingpin of the semitrailer from the fifth wheel of the truck tractor by disengaging the fifth wheel jaws. Since the tractor is spring-biased in an upward direction, it is desirable to raise the semitrailer to relieve some of the load allowing the tractor to pull away from the semitrailer more easily. Therefore, the operator may shift the gear assembly to low gear in order to further extend the landing gear legs and raise all or part of the load of the semitrailer from the fifth wheel. After releasing the movable jaws of the fifth wheel, the tractor is disengaged from the semitrailer.
When an operator picks up a semitrailer which has been loaded or unloaded, he or she must back the truck tractor fifth wheel under the semitrailer and engage the kingpin. If the semitrailer was resting on a stable surface such as concrete, and a similar tractor is utilized, the necessary clearance to allow coupling most likely still exists between the ground and the kingpin, and the operator merely will back the truck tractor under the semitrailer engaging the kingpin. The operator will shift the landing gear into low gear to lower the load onto the tractor, then shift into high gear and quickly retract the legs to provide clearance for vehicle operation. More particularly, the legs are positioned approximately one foot off of the ground to provide clearance for over-the-road travel. However, if the semitrailer was left on a relatively unstable surface such as soft ground or blacktop, the semitrailer legs sometimes sink into the ground under the load of the trailer between the drop off and pickup times. If this is the case, or if a higher tractor is employed, prior to backing the truck tractor under the semitrailer the operator must further extend the legs to raise the trailer to provide sufficient clearance between the ground and the kingpin. This requires the operator to shift the landing gear into low gear to gain mechanical advantage to raise the trailer. After coupling to the tractor and lowering the trailer load onto the tractor using low gear, the operator then will proceed as described above to retract the legs in high gear to position the legs for over-the-road travel.
As expected, in view of the potential income accruing from possible sales of landing gear units, manufacturers of the landing gear are continually attempting to improve their products to present a more economical landing gear having improved performance features, in order to gain a larger share of the landing gear market.